1. Field of the Invention
This invention relates generally to solid-state thermionic heating and cooling devices for thermally treating the human body, and more particularly to such a device that has a solidstate thermionic thermal diode unit with a carbon foam heat sink that provides greater surface area per unit volume and significantly greater heat transfer efficiency than conventional thermoelectric Peltier devices with finned heat sinks and facilitates miniaturization.
2. Description of the Related Art
Peltier effect thermoelectric heat pumping devices that are used for thermally treating (cooling or heating) the human body are known in the art. Such solid-state heat pumping devices typically contain a Peltier module, a heat sink and a fan. Most prior art Peltier devices utilize a heat sink that is comprised of a block of aluminum or copper having protruding fins from which heat is removed by radiation, natural convection or the forced convection of a fan. These types of prior art heat sinks are bulky and heavy and generally constitute the largest component of the heat pump device and thereby render the devices unsatisfactory for applications having size or weight restrictions. A fan placed in close proximity of the heat sink also adds to the size of the device.
Klein, U.S. Pat. No. 4,930,317 discloses a hot/cold therapy device having a hot/cold pad assembly remotely connected to a control module. The hot/cold pad assembly includes a flexible pad filled with a convection (gel) or conduction (laminated metal, rubber impregnated with metal particles, etc.), or combination thereof (a conductive metal layer immersed in a gel). A thin plate of conductive material (copper) forms an extension of the cold plate of a thermoelectric heat pump, both of which are in thermal contact with the pad. A finned air-cooled heat sink is mounted in a housing or shroud which is connected by a flexible umbilical line to the control module. The control module includes a fan for drawing or blowing air through the shroud to maintain the temperature difference between the finned heat sink and attached plate of the thermoelectric heat pump element.
Galvan et al, U.S. Pat. No. 5,197,294 discloses a miniaturized thermoelectric Peltier apparatus for air conditioning a protective body suit. The Peltier device, in the form of bimetallic or plurimetallic plates is connected to a low voltage D.C. power supply, the opposed cold and hot surface of which are in contact with respective finned heat exchangers. The assembly is contained in a housing in which two distinct and separate conduits are provided for the forced flow of air through the respective ones of the finned heat exchangers.
Arnold, U.S. Pat. No. 5,970,718 discloses a personal heat control apparatus having an outer casing that accommodates a Peltier-effect unit, one or more batteries and a timing switch for selective energization of the unit. The casing is releasably attached to a part of a person's body, e.g. the wrist, by a strap with a cooling surface cooled by the unit in contact with the body part to enhance heat transfer between the person's body and the surrounding air for comfort and refreshment purposes when the unit is energized. Heat generated in the unit is dispersed through the outer casing, which serves as the heat sink. The surface area of the heat sink (casing) may be several times the size of the cooling surface, e.g. five times larger, to promote rapid heat dissipation.
Johnston, U.S. Pat. No. 6,023,932 discloses a portable topical heat transfer device for topically cooling an animal or human which comprises a thermoelectric unit having a cold side and a warm side, a DC source which is connected to the thermoelectric unit, a finned heat sink which is mounted in a heat conductive relationship with the warm side of the thermoelectric unit, a fan for removing heat from the heat sink, and a strap or the like for securing the device to the body of the wearer. The heat sink comprises the main body portion of the device and includes a series of fins and channels.
Taylor et al, U.S. Pat. No. 6,125,636 discloses a self-contained personal cooling and/or heating device that includes a heat-dissipating member which fits around a portion of a user's body. A Peltier thermo-voltaic module operated with low voltage at relatively low current is thermally coupled to the rear surface of the member, and the rear surface of the module is provided with a large surface area, preferably augmented by a finned heat sink. A fan directs ambient air onto this rear module surface or heat sink.
Most of these prior art devices are unsatisfactory for personal cooling applications having size or weight restrictions because they utilize less efficient thermoelectric Peltier modules with one or more finned heat sinks formed of aluminum or copper which are bulky and heavy and generally constitute the largest component of the device, or they utilize a casing or housing of relatively large surface area that serves to dissipate heat. Some of these devices also utilize a fan in close proximity to the heat sink, which also adds to the size of the device. Recently developed “thermionic” temperature modification devices, also known as “vacuum diode heat pumps” or “thermal diodes”, are solid-state semiconductor devices capable of alternating between heating and cooling of a particular element. Thermionic devices are similar in concept to thermoelectric (Peltier) devices, except that the thermal diode operates in the manner of a vacuum diode wherein the electrons move across a gap, which is an excellent insulator. The gap may also be evacuated to create a vacuum. With the addition of a voltage bias that encourages the electrons to move in a given direction, the heat is transferred from one side (cathode or emitter) to the other (anode or collector) across the gap between the two materials, and the gap prevents the heat from flowing back.
Tunneling diodes or resonant tunneling diodes (RTD) are small semiconductor chips that utilize ultra-thin layers wherein a “well” of silicon or other material is sandwiched between two barrier layers (for example silicon dioxide) with electrical contacts on the top and bottom, wherein, upon application of a voltage bias, electrons “tunnel” from one side of the barrier to the other. This movement is known as “tunneling”. Resonant tunneling diodes (RTD) have also been made by growing extremely thin layers of chemically different semiconductors (such as gallium arsenide, GaAs, and aluminum arsenide, AlAs) on top of one another in a single crystal structure, wherein the layer of GaAs between layers of AlAs may be a few nanometers thick to form a well of finite depth for electrons.
Cool Chips plc., a subsidiary of Borealis Exploration Limited (BOREF), based in Gibralter, has recently developed a form of vacuum diode known as Cool Cool Chips™ that pump heat from one side of the chip to the other to provide localized cooling and refrigeration utilizing a “thermotunnel” technology based on known principles of quantum physics wherein electrons are made to “tunnel” from one side of a tiny vacuum gap (50 nanometers or less) to another. These devices are reported to deliver up to a projected 70-80% of the maximum theoretical efficiency for a perfect heat pump (known as Carnot Efficiency). The Borealis Cool Cool Chips™ and “thermotunnel” technology are disclosed in U.S. Pat. No. 6,417,060 and U.S. Pat. No. 6,285,769, which are hereby incorporated by reference to the same extent as if fully set forth herein.
Thermoelectric (Peltier) devices, on the other hand, do not have a gap and the electrons that carry heat in one direction flow through the material itself and most of that heat is returned through conduction. Conventional thermoelectric (Peltier) devices may achieve an efficiency of up to 5-8% of the theoretical maximum for a perfect heat pump (Carnot Efficiency). Thus, thermionic vacuum diode heat pump and thermal diode devices offer much higher efficiency, higher output cooling in a much smaller package, and require less power to operate, than is possible with thermoelectric (Peltier) devices.
U.S. Pat. No. 5,722,242 and U.S. Pat. No. 6,396,191, hereby incorporated by reference to the same extent as if fully set forth herein, teach the construction and operation of a vacuum diode heat pump, and a thermal diode, respectively. U.S. Pat. No. 6,417,060, hereby incorporated by,reference, discloses methods for making diode devices and electrodes that may be used in a diode device such as thermionic converters and generators, photoelectric converters and generators, vacuum diode heat pumps, and thermotunnel converters.
Although not a personal cooling device for thermally treating the human body, Klett et al, U.S. Pat. No. 6,430,935 discloses an air cooling filtering device for providing cool or warm filtered air for a person to breathe that includes at least one thermoelectric and/or thermionic element and at least one thermally conductive carbon foam element (described in detail hereinafter), which has at least one flow channel through which fluids (air) passes. The thermoelectric and/or thermionic device is thermally connected to the carbon foam element and modifies the temperature of the carbon foam, which in turn, modifies the temperature of air flowing through the flow channels. In a preferred embodiment, the thermoelectric and/or thermionic device is sandwiched between two carbon foam elements. This device does not place the cold or hot surface of the thermoelectric and/or thermionic member in thermal contact with the object to be cooled or heated, but instead teaches placing the foam material in thermal contact with the object to be cooled or heated (air). The thermoelectric and/or thermionic device merely serves to modify the temperature of the foam, and the object to be cooled or heated (air) passes from an inlet end to an outlet end of the flow channels in the foam, rather than being drawn through the cellular structure of the foam medium.
The carbon foam heat sink material utilized in U.S. Pat. No. 6,430,935 and also preferably in the present invention is a recently developed new material is produced by a patented foaming process disclosed in U.S. Pat. Nos. 6,033,506 and 6,037,032 issued to James W. Klett et al., and which are hereby incorporated by reference to the same extent as if fully set forth herein. This carbon foam material is commercially known as “PocoFoam”® marketed by Poco Graphite, Inc., of 1601 South Street, Decatur, Tex. 76234.
This new material is high thermal conductivity porous foam which allows the transfer of a large volume of thermal energy. The carbon or graphite aligned ligament structure conducts heat better than copper or aluminum. Its porous structure gives it an enormous surface area, such as two square meters per cubic centimeter of material. That is to say, a sugar cube sized piece of this material would have the surface area of forty-two square feet and weigh half a gram. Air is passed through its interconnecting porous network wherein heat is removed. In comparison, a sugar cube sized aluminum heat sink may have a surface area of a few square inches and may only transfer a minute amount of heat.
The present invention overcomes the size and weight limitations of the prior art and is distinguished over the prior art in general, and these patents in particular, by a miniaturized solid-state thermionic heating and cooling device for thermally treating the human body that utilizes thermionic thermal diodes which provides high efficiency, high output cooling in a very small package, and a carbon foam heat sink of porous open cell structure which provides greater surface area per unit volume and significantly greater heat transfer efficiency than conventional finned heat sinks. Miniaturization is further facilitated by placing the air pump and power supply unit remote from the thermionic heating and cooling unit to extract and dispose of heat collected from the carbon foam heat sink.